Molecular resist composition and patterning process

ABSTRACT

A molecular resist composition comprising a sulfonium salt having formula (1) or (2) and an organic solvent has a high sensitivity and forms a resist film with improved resolution and LWR, when processed by EB or EUV lithography.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2021-179526 filed in Japan on Nov. 2, 2021, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a molecular resist composition and a pattern forming process.

BACKGROUND ART

While a higher integration density, higher operating speed and lower power consumption of LSIs are demanded to comply with the expanding IoT market, the effort to reduce the pattern rule is in rapid progress. The wide-spreading logic device market drives forward the miniaturization technology. As the advanced miniaturization technology, microelectronic devices of 10-nm node are manufactured in a mass scale by the double, triple or quadro-patterning version of the ArF immersion lithography. The manufacture of 7-nm node devices by the next generation EUV lithography of wavelength 13.5 nm is investigated.

In the EUV lithography, chemically amplified resist compositions are applicable to form line patterns to a line width of 20 nm or less. When a polymeric resist composition commonly used in the ArF lithography is used in the EUV lithography, such problems as roughening of pattern surface and difficulty of pattern control arise because the base polymer in the composition has a large molecular size. To overcome the problems, a variety of low-molecular-weight materials are proposed.

Molecular resist compositions are based on low-molecular-weight compounds and free of base polymers commonly used in polymeric resist compositions. The molecular resist composition is expected as one of effective measures for forming small-size patterns. For example, Patent Document 1 discloses a negative tone radiation-sensitive composition adapted for alkaline development, comprising mainly a polyhydric polyphenol compound. Non-Patent Document 1 describes a positive tone resist composition adapted for alkaline development, comprising only an acid generator in the form of a sulfonium salt in which a cation having a tert-butoxycarbonyloxy group attached thereto is combined with a strong acid anion. Since the acid generator has a smaller molecular size than polymers, an improvement in roughness is expectable. However, since the molecular resist composition relying on the chemical amplification mechanism is difficult to control acid diffusion, no satisfactory performance has yet been established. In addition, the EUV resist composition must not only clear roughness, but also provide high sensitivity and resolution at the same time, with further improvements being desired.

One of the causes that retard the development of EUV lithography materials is a small number of photons available with EUV exposure. The energy of EUV is extremely higher than that of ArF excimer laser. The number of photons available with EUV exposure is 1/14 of the number by ArF exposure. The size of pattern features formed by the EUV lithography is less than half the size by the ArF lithography. Therefore, the EUV lithography is quite sensitive to a variation of photon number. A variation in number of photons in the radiation region of extremely short wavelength is shot noise as a physical phenomenon. It is impossible to eliminate the influence of shot noise. Attention is thus paid to stochastics. While it is impossible to eliminate the influence of shot noise, discussions are held how to reduce the influence. There is observed a phenomenon that under the influence of shot noise, values of CDU and LWR are increased and holes are blocked at a probability of one several millionth. The blockage of holes leads to electric conduction failure to prevent transistors from operation, adversely affecting the performance of an overall device.

As the means for reducing the influence of shot noise on the resist side, Patent Document 2 discloses an inorganic resist composition comprising a complex of an element having high EUV absorption. Although the inorganic resist composition has a relatively high sensitivity, it is not yet satisfactory because of outstanding problems including poor solubility in resist solvents, low shelf stability, and defectiveness.

CITATION LIST

-   Patent Document 1: JP-A 2005-326838 (U.S. Pat. No. 7,871,751) -   Patent Document 2: JP-A 2015-108781 (U.S. Pat. No. 9,366,960) -   Non-Patent Document 1: Proc. of SPIE Vol. 6923, 69230K (2008)

DISCLOSURE OF INVENTION

An object of the invention is to provide a molecular resist composition which is improved in sensitivity, resolution, and LWR when processed by photolithography using high-energy radiation, especially EB or EUV lithography; and a pattern forming process using the resist composition.

The inventors have found that a molecular resist composition comprising a sulfonium salt having a cation of specific partial structure has a high sensitivity and forms a resist film with improved resolution and LWR, so that the resist composition is quite useful for precise micropatterning.

In one aspect, the invention provides a molecular resist composition comprising a sulfonium salt having the formula (1) or (2) and an organic solvent, the composition being free of a base polymer.

Herein n is an integer of 1 to 3,

A¹ is a polymerizable functionality-bearing C₂-C₂₀ hydrocarbyl group which may contain a heteroatom,

A² is a polymerizable functionality-bearing group which forms a C₄-C₂₀ aliphatic ring with two carbon atoms in Ar^(1B), and the aliphatic ring may contain a heteroatom,

Ar^(1A) is a C₆-C₂₀ arylene group in which some or all of the hydrogen atoms on the aromatic ring may be substituted by halogen or a C₁-C₂₀ hydrocarbyl moiety which may contain a heteroatom,

Ar^(1B) is a C₆-C₂₀ trivalent aromatic hydrocarbon group in which some or all of the hydrogen atoms on the aromatic ring may be substituted by halogen or a C₁-C₂₀ hydrocarbyl moiety which may contain a heteroatom,

Ar² is a C₆-C₂₀ aryl group in which some or all of the hydrogen atoms on the aromatic ring may be substituted by halogen or a C₁-C₂₀ hydrocarbyl moiety which may contain a heteroatom,

two Ar^(1A), two Ar^(1B), two Ar², Ar^(1A) and Ar², or Ar^(1B) and Ar² may bond together to form a ring with the sulfur atom to which they are attached, and

X⁻ is a counter anion.

In a preferred embodiment, A¹ is acryloyloxy, methacryloyloxy, a C₃-C₂₀ cycloalkenyl group, C₃-C₂₀ cycloalkenyloxy group, C₃-C₂₀ cycloalkenylcarbonyloxy group, C₂-C₂₀ alkenyl group, or C₂-C₂₀ alkenyloxy group, the cycloalkenyl, cycloalkenyloxy, cycloalkenylcarbonyloxy, alkenyl and alkenyloxy groups may contain a heteroatom,

A² is a group which forms a C₄-C₂₀ cycloalkene ring or C₄-C₂₀ multiple ring having one double bond, with two carbon atoms in Ar^(1B), the cycloalkene ring and multiple ring may contain a heteroatom.

In a preferred embodiment, X⁻ is a halide ion, nitrate ion, hydrogensulfate ion, hydrogencarbonate ion, tetraphenylborate ion, or an anion having any one of the formulae (X-1) to (X-7).

Herein k is an integer of 1 to 4,

Rf¹ and Rf² are each independently hydrogen, fluorine or a C₁-C₆ fluorinated alkyl group, excluding that all Rf¹ and Rf² are hydrogen at the same time,

R¹¹ is hydrogen, halogen, hydroxy, or a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom,

R¹² is hydrogen, halogen, hydroxy, or a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom, exclusive of a hydrocarbyl group in which hydrogen on α- or β-carbon relative to the sulfo group is substituted by fluorine or fluoroalkyl,

R²¹ is hydrogen, halogen, hydroxy, or a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom,

R²² is hydrogen, halogen, hydroxy, or a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom, exclusive of a hydrocarbyl group in which hydrogen on α- or β-carbon relative to the carboxy group is substituted by fluorine or fluoroalkyl,

R³¹ and R³² are each independently a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom,

R⁴¹ to R⁴³ are each independently a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom,

R⁵¹ is fluorine or a C₁-C₁₀ fluorinated hydrocarbyl group which may contain hydroxy, ether bond or ester bond, R⁵² is hydrogen or a C₁-C₂₀ hydrocarbyl group which may contain hydroxy, ether bond or ester bond, R⁵¹ and R⁵² may bond together to form a ring with the atoms to which they are attached.

In a preferred embodiment, the anion having any one of formulae (X-1) to (X-7) contains a polymerizable functional group.

The molecular resist composition may further comprise a radical trapping agent and/or a surfactant.

In another aspect, the invention provides a pattern forming process comprising the steps of applying the molecular resist composition defined herein onto a substrate to form a resist film thereon, exposing the resist film to high-energy radiation, and developing the exposed resist film in a developer.

In a preferred embodiment, the developing step uses an alkaline aqueous solution as the developer to form a positive tone pattern wherein the resist film in the exposed region is dissolved away and the resist film in the unexposed region is not dissolved.

In another preferred embodiment, the developing step uses an organic solvent as the developer to form a negative tone pattern wherein the resist film in the unexposed region is dissolved away and the resist film in the exposed region is not dissolved.

The organic solvent is preferably selected from among 2-octanone, 2-nonanone, 2-heptanone, 3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone, diisobutyl ketone, methylcyclohexanone, acetophenone, methylacetophenone, propyl acetate, butyl acetate, isobutyl acetate, pentyl acetate, butenyl acetate, isopentyl acetate, propyl formate, butyl formate, isobutyl formate, pentyl formate, isopentyl formate, methyl valerate, methyl pentenoate, methyl crotonate, ethyl crotonate, methyl propionate, ethyl propionate, ethyl 3-ethoxypropionate, methyl lactate, ethyl lactate, propyl lactate, butyl lactate, isobutyl lactate, pentyl lactate, isopentyl lactate, methyl 2-hydroxyisobutyrate, ethyl 2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, phenyl acetate, benzyl acetate, methyl phenylacetate, benzyl formate, phenylethyl formate, methyl 3-phenylpropionate, benzyl propionate, and 2-phenylethyl acetate.

Typically, the high-energy radiation is EB or EUV.

Advantageous Effects of Invention

The molecular resist composition of the invention meets both high sensitivity and high resolution and is improved in LWR when processed by photolithography using high-energy radiation, especially EB or EUV lithography. The resist composition is quite useful for precise micropatterning.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the ¹H-NMR spectrum of PAG-1 in Synthesis Example 1-1.

FIG. 2 is a diagram showing the ¹H-NMR spectrum of PAG-2 in Synthesis Example 1-2.

FIG. 3 is a diagram showing the ¹H-NMR spectrum of PAG-3 in Synthesis Example 1-3.

FIG. 4 is a diagram showing the ¹H-NMR spectrum of PAG-4 in Synthesis Example 1-4.

FIG. 5 is a diagram showing the ¹H-NMR spectrum of PAG-5 in Synthesis Example 1-5.

FIG. 6 is a diagram showing the ¹H-NMR spectrum of PAG-6 in Synthesis Example 1-6.

FIG. 7 is a diagram showing the ¹H-NMR spectrum of PAG-7 in Synthesis Example 1-7.

FIG. 8 is a diagram showing the ¹H-NMR spectrum of PAG-8 in Synthesis Example 1-8.

FIG. 9 is a diagram showing the ¹H-NMR spectrum of PAG-9 in Synthesis Example 1-9.

DESCRIPTION OF EMBODIMENTS

As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. The notation (Cn-Cm) means a group containing from n to m carbon atoms per group. In chemical formulae, the broken line designates a valence bond. The terms “group” and “moiety” are interchangeable. Me stands for methyl and Ac stands for acetyl.

The abbreviations and acronyms have the following meaning.

-   -   EB: electron beam     -   EUV: extreme ultraviolet     -   PEB: post-exposure bake     -   PAG: photoacid generator     -   LWR: line width roughness     -   CDU: critical dimension uniformity

Sulfonium Salt

The invention provides a molecular resist composition comprising a sulfonium salt having the following formula (1) or (2) as a main component. As used herein, the main component means that the amount of this component is the most in the composition except an organic solvent.

In formulae (1) and (2), n is an integer of 1 to 3.

In formula (1), A¹ is a C₂-C₂₀ hydrocarbyl group containing a polymerizable functional moiety, and the hydrocarbyl group may contain a heteroatom.

In formula (2), A² is a group which forms a C₄-C₂₀ aliphatic ring with two carbon atoms in Ar^(1B), the group contains a polymerizable functional moiety, and the aliphatic ring may contain a heteroatom. Suitable heteroatoms include oxygen, sulfur, nitrogen and halogen atoms.

Preferably, A¹ is acryloyloxy, methacryloyloxy, a C₃-C₂₀ cycloalkenyl group, C₃-C₂₀ cycloalkenyloxy group, C₃-C₂₀ cycloalkenylcarbonyloxy group, C₂-C₂₀ alkenyl group, or C₂-C₂₀ alkenyloxy group. The cycloalkenyl, cycloalkenyloxy, cycloalkenylcarbonyloxy, alkenyl, and alkenyloxy groups each may contain a heteroatom. Also preferably, A² is a group which forms a C₄-C₂₀ cycloalkene ring or C₄-C₂₀ multiple ring having one double bond, with two carbon atoms in Ar^(1B). The cycloalkene ring and multiple ring each may contain a heteroatom.

Examples of the group represented by A¹ are shown below, but not limited thereto. Herein, the broken line designates a point of attachment to Ar¹.

When A² is a group which forms a C₄-C₂₀ aliphatic ring with two carbon atoms in Ar^(1B), examples of the aliphatic ring are shown below, but not limited thereto. Herein, “C” designates a carbon atom in Ar^(1B).

Of these, A-1 to A-18, A-30 to A-43, and A-57 are preferred in view of ease of introduction and polymerization reactivity in the synthesis course, with A-9 to A-18, A-30 to A-35, and A-40 being more preferred.

In formula (1), Ar^(1A) is a C₆-C₂₀ arylene group. Exemplary of the arylene group are phenylene, naphthylene and anthracenediyl. In formula (2), Ar^(1B) is a C₆-C₂₀ trivalent aromatic hydrocarbon group. Exemplary of the trivalent aromatic hydrocarbon group are groups obtained by removing three hydrogen atoms from benzene, naphthalene and anthracene. In formulae (1) and (2), Ar² is a C₆-C₂₀ aryl group. Exemplary of the aryl group are phenyl, naphthyl and anthracenyl. Of these, in view of solvent solubility, Ar^(1A) is preferably phenylene or naphthylene, more preferably phenylene; Ar^(1B) is preferably benzenetriyl or naphthalenetriyl, more preferably benzenetriyl; and Ar² is preferably phenyl or naphthyl, more preferably phenyl.

In the arylene group, trivalent aromatic hydrocarbon group, and aryl group, some or all of the hydrogen atoms may be substituted by halogen or a C₁-C₂₀ hydrocarbyl group which may contain a heteroatom. Suitable halogen atoms include fluorine, chlorine, bromine, and iodine. The C₁-C₂₀ hydrocarbyl group may be saturated or unsaturated and straight, branched or cyclic. Examples thereof include C₁-C₂₀ alkyl groups such as methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, tert-pentyl, n-pentyl, n-hexyl, n-octyl, 2-ethylhexyl, n-nonyl and n-decyl; C₃-C₂₀ cyclic saturated hydrocarbyl groups such as cyclopentyl, cyclohexyl, cyclopentylmethyl, cyclopentylethyl, cyclopentylbutyl, cyclohexylmethyl, cyclohexylethyl, cyclohexylbutyl, norbornyl, tricyclo[5.2.1.0^(2,6)]decanyl, adamantyl, and adamantylmethyl; C₆-C₂₀ aryl groups such as phenyl, naphthyl and anthracenyl. In the hydrocarbyl group, some or all of the hydrogen atoms may be substituted by a moiety containing a heteroatom such as oxygen, sulfur, nitrogen or halogen, and some constituent —CH₂— may be replaced by a moiety containing a heteroatom such as oxygen, sulfur or nitrogen, so that the group may contain a hydroxy, cyano, halogen, carbonyl, ether bond, thioether bond, ester bond, sulfonic ester bond, carbonate bond, carbamate bond, lactone ring, sultone ring or carboxylic anhydride (—C(═O)—O—C(═O)—).

Two Ar^(1A), two Ar^(1B), two Ar², a pair of Ar^(1A) and Ar², or a pair of Ar^(1B) and Ar² may bond together to form a ring with the sulfur atom to which they are attached. Suitable ring structures are shown below, but not limited thereto.

Examples of the cation in the sulfonium salt having formula (1) are shown below, but not limited thereto.

Examples of the cation in the sulfonium salt having formula (2) are shown below, but not limited thereto.

In formulae (1) and (2), X⁻ is a counter anion. The preferred counter anion is selected from halide ions, nitrate ions, hydrogensulfate ions, hydrogencarbonate ions, tetraphenylborate ions, and anions having the formulae (X-1) to (X-7).

In formulae (X-1) and (X-3), k is an integer of 1 to 4. Rf¹ and Rf² are each independently hydrogen, fluorine or a C₁-C₆ fluorinated alkyl group. Not all Rf¹ and Rf² are hydrogen at the same time.

In formula (X-1), R¹¹ is hydrogen, halogen, hydroxy, or a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom.

In formula (X-2), R¹² is hydrogen, halogen, hydroxy, or a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom, exclusive of a hydrocarbyl group in which hydrogen on α- or β-carbon relative to the sulfo group is substituted by fluorine or fluoroalkyl.

In formula (X-3), R²¹ is hydrogen, halogen, hydroxy, or a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom.

In formula (X-4), R²² is hydrogen, halogen, hydroxy, or a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom, exclusive of a hydrocarbyl group in which hydrogen on α- or β-carbon relative to the carboxy group is substituted by fluorine or fluoroalkyl.

In formula (X-5), R³¹ and R³² are each independently a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom.

In formula (X-6), R⁴¹ to R⁴³ are each independently a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom.

In formula (X-7), R⁵¹ is fluorine or a C₁-C₁₀ fluorinated hydrocarbyl group which may contain hydroxy, ether bond or ester bond. R⁵² is hydrogen or a C₁-C₂₀ hydrocarbyl group which may contain hydroxy, ether bond or ester bond. R⁵¹ and R⁵² may bond together to form a ring with the atoms to which they are attached.

As the anion X⁻, preference is given to halide ions, nitrate ions, and anions having formulae (X-1) to (X-7), especially halide ions, nitrate ions, and anions having formulae (X-2), (X-4) and (X-6).

The C₁-C₄₀ hydrocarbyl group represented by R¹¹, R¹², R²¹, R²², R³¹, R³², R⁴¹, R⁴² and R⁴³ may be saturated or unsaturated and straight, branched or cyclic. Examples thereof include C₁-C₄₀ alkyl groups such as methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, tert-pentyl, n-pentyl, n-hexyl, n-octyl, 2-ethylhexyl, n-nonyl and n-decyl; C₃-C₄₀ cyclic saturated hydrocarbyl groups such as cyclopentyl, cyclohexyl, cyclopentylmethyl, cyclopentylethyl, cyclopentylbutyl, cyclohexylmethyl, cyclohexylethyl, cyclohexylbutyl, norbornyl, tricyclo[5.2.1.0^(2,6)]decanyl, adamantyl, and adamantylmethyl; C₆-C₄₀ aryl groups such as phenyl, naphthyl, and anthracenyl; and combinations thereof. In the hydrocarbyl group, some or all hydrogen may be substituted by a moiety containing a heteroatom such as oxygen, sulfur, nitrogen or halogen, or some constituent —CH₂— may be replaced by a moiety containing a heteroatom such as oxygen, sulfur or nitrogen, so that the group may contain a hydroxy, cyano, halogen, carbonyl, ether bond, thioether bond, ester bond, sulfonic ester bond, carbonate bond, carbamate bond, lactone ring, sultone ring or carboxylic anhydride (—C(═O)—O—C(═O)—).

The C₁-C₁₀ fluorinated hydrocarbyl group represented by R⁵¹ is a C₁-C₁₀ hydrocarbyl group in which some or all hydrogen atoms are substituted by fluorine. The C₁-C₁₀ hydrocarbyl group may be saturated or unsaturated and straight, branched or cyclic, and examples thereof are as exemplified above for the C₁-C₄₀ hydrocarbyl group represented by R¹¹, R¹², R²¹, R²², R³¹, R³², R⁴¹, R⁴² and R⁴³, but of 1 to 10 carbon atoms.

The C₁-C₂₀ hydrocarbyl group represented by R⁵² may be saturated or unsaturated and straight, branched or cyclic, and examples thereof are as exemplified above for the C₁-C₄₀ hydrocarbyl group represented by R¹¹, R¹², R²¹, R²², R³¹, R³², R⁴¹, R⁴² and R⁴³, but of 1 to 20 carbon atoms.

The anion having any one of formulae (X-1) to (X-7) may possess a C₂-C₂₀ hydrocarbyl group which contains a polymerizable functional group in its structure and may contain a heteroatom. Examples thereof are the same as exemplified above for the group A¹ in formula (1).

Examples of the anion having formula (X-1) are shown below, but not limited thereto. Herein Rf¹ is as defined above.

Examples of the anion having formula (X-2) are shown below, but not limited thereto.

Examples of the anion having formula (X-3) are shown below, but not limited thereto.

Examples of the anion having formula (X-4) are shown below, but not limited thereto.

Examples of the anion having formula (X-5) are shown below, but not limited thereto.

Examples of the anion having formula (X-6) are shown below, but not limited thereto.

Examples of the anion having formula (X-7) are shown below, but not limited thereto.

Suitable examples of the sulfonium salt having formula (1) or (2) include arbitrary combinations of anions with cations, both as exemplified above.

For the sulfonium salt having formula (1) or (2), it is preferred from the aspect of pattern formation that both the cation and the anion of the salt contain a polymerizable functional group.

The sulfonium salt having formula (1) or (2) may be used alone or in admixture of two or more. From the standpoint of enhancing the uniformity of component, it is preferred to use the sulfonium salt alone or as a mixture of two.

The sulfonium salt having formula (1) or (2) may be synthesized by a suitable combination of well-known organic chemistry procedures. One exemplary procedure is by mixing an onium salt intermediate having a desired cation with an onium salt intermediate having a desired anion, followed by ion exchange reaction. The ion exchange reaction may be performed by a well-known technique, for example, with reference to JP-A 2007-145797.

Organic Solvent

The molecular resist composition of the invention contains an organic solvent. The solvent used herein is not particularly limited as long as it dissolves the sulfonium salt having formula (1) or (2) and enables film formation. Suitable organic solvents include ketones such as cyclohexanone and methyl-2-n-pentyl ketone; alcohols such as 3-methoxybutanol, 3-methyl-3-methoxybutanol, 1-methoxy-2-propanol, 1-ethoxy-2-propanol, and diacetone alcohol (DAA); ethers such as propylene glycol monomethyl ether, ethylene glycol monomethyl ether, propylene glycol monoethyl ether, ethylene glycol monoethyl ether, propylene glycol dimethyl ether, and diethylene glycol dimethyl ether; esters such as propylene glycol monomethyl ether acetate (PGMEA), propylene glycol monoethyl ether acetate, ethyl lactate, ethyl pyruvate, butyl acetate, methyl 3-methoxypropionate, ethyl 3-ethoxypropionate, tert-butyl acetate, tert-butyl propionate, and propylene glycol mono-tert-butyl ether acetate; and lactones such as γ-butyrolactone (GBL).

Of these, 1-ethoxy-2-propanol, PGMEA, cyclohexanone, DAA, GBL, and mixtures thereof are preferred.

An appropriate amount of the organic solvent used is 200 to 5,000 parts by weight per 100 parts by weight of the sulfonium salt having formula (1) or (2). The organic solvent may be used alone or in admixture.

The molecular resist composition of the invention is characterized by comprising the sulfonium salt having formula (1) or (2) as a main component and the organic solvent, and being free of a base polymer. When a resist film formed from the molecular resist composition is exposed to EB or EUV, it forms a negative tone pattern as a result of the resist film in the exposed region turning insoluble in alkaline developer. As used herein, the term “base polymer” refers to a polymer which is a main component of polymeric resist compositions and adapted to change its solubility in developer under the action of an acid generated from an acid generator.

In resist compositions of conventional design, that is, comprising a multi-component polymer as a main component or base polymer, a photoacid generator, a sensitivity adjusting agent and other additives, it is unlikely that the components are uniformly distributed in a resist film. The non-uniform distribution has a substantial impact on roughness particularly in forming small-size patterns by the EUV lithography. This, combined with the additional influence of the polymer having a large molecular size, causes to degrade LWR and CDU.

In contrast, the molecular resist composition is improved in the uniform distribution of components in a resist film because the composition does not contain a multi-component polymer and is of quite simple design. The main component is a low-molecular-weight compound having a small molecular size. LWR and CDU are thus improved particularly in forming small-size patterns by the EB or EUV lithography.

The molecular resist composition enables pattern formation by utilizing a structural change as a result of photo-reaction of the sulfonium salt as the main component and polymerization of the polymerizable functional group. On use of the sulfonium salt having formula (1) or (2), photo-decomposition of the sulfonium salt in the EB or EUV lithography and radical polymerization of a polymerizable group induced by a radical generated upon light exposure take place, which brings about a substantial change of solubility in alkaline developer (i.e., insolubilization), forming a negative tone pattern. Particularly on use of the sulfonium salt whose cation and anion both contain a polymerizable functional group, all of the constituents of the salt contribute to pattern formation during radical polymerization upon light exposure, which is effective for enhancing the dissolution contrast between exposed and unexposed regions. Since the structural change takes place during light exposure, acid diffusion as observed in conventional polymeric chemically amplified resist compositions does not occur, that is, image blur due to acid diffusion is eliminated. The molecular resist composition shows better resolution performance than conventional polymeric chemically amplified resist compositions based on a polymer and is fully resistant to pattern collapse. The molecular resist composition is thus quite effective in forming small-size patterns.

Although the molecular resist composition does not contain a polymer component functioning as a base polymer, it may contain a polymer component which is used as an additive or which is not a main component, like a polymer serving as a surfactant, as long as the pattern formation by the sulfonium salt having formula (1) or (2) is not hindered.

Other Components

The molecular resist composition may contain a radical trapping agent as another component. The addition of a radical trapping agent enables to control photo-reaction during photolithography and adjust sensitivity.

Suitable radical trapping agents include hindered phenols, quinones, hindered amines, and thiol compounds. Exemplary hindered phenols include dibutylhydroxytoluene (BHT) and 2,2′-methylenebis(4-methyl-6-tert-butylphenol). Exemplary quinones include 4-methoxyphenol (e.g., Methoquinone) and hydroquinone. Exemplary hindered amines include 2,2,6,6-tetramethylpiperidine and 2,2,6,6-tetramethylpiperidine-N-oxy radical. Exemplary thiol compounds include dodecane thiol, hexadecane thiol and benzene thiol. When the molecular resist composition contains a radical trapping agent, the amount of the agent is preferably 0.1 to 20 parts, more preferably 0.5 to 10 parts by weight per 100 parts by weight of the sulfonium salt. The radical trapping agent may be used alone or in admixture.

The molecular resist composition may contain a surfactant as a further component. Suitable surfactants include FC-4432 and FC-4430 (3M), PF636, PF656, PF6320, and PF6520 (Omnova Solutions Inc.). When the molecular resist composition contains a surfactant, the amount of the surfactant is preferably 0.001 to 20 parts, more preferably 0.1 to 10 parts by weight per 100 parts by weight of the sulfonium salt. The surfactant may be used alone or in admixture.

Process

Another embodiment of the invention is a pattern forming process using the molecular resist composition defined above. A variety of integrated circuits may be formed from the resist composition using any well-known lithography process. The preferred process includes the steps of applying the molecular resist composition to a substrate to form a resist film thereon, exposing the resist film to high-energy radiation, and developing the exposed resist film in a developer. Any desired steps may be added to the process if necessary.

First, the resist composition is applied to a substrate by a suitable coating technique such as spin coating, roll coating, flow coating, dipping, spraying or doctor coating. The substrate used herein may be a substrate for integrated circuitry fabrication, e.g., Si, SiO₂, SiN, SiON, TiN, WSi, BPSG, SOG, organic antireflective film, etc. or a substrate for mask circuitry fabrication, e.g., Cr, CrO, CrON, MoSi2, SiO₂, etc. The coating is prebaked on a hot plate preferably at a temperature of 60 to 150° C. for 10 seconds to 30 minutes, more preferably at 80 to 120° C. for 30 seconds to 20 minutes. The resulting resist film preferably has a thickness of 0.01 to 2 μm.

Then the resist film is exposed patternwise to high-energy radiation. Examples of the high-energy radiation include UV, deep UV, EB, EUV, X-ray, soft X-ray, excimer laser, γ-ray, and synchrotron radiation. On use of UV, deep UV, EUV, X-ray, soft X-ray, excimer laser, γ-ray or synchrotron radiation, the resist film is exposed directly or through a mask having a desired pattern, preferably in a dose of 1 to 200 mJ/cm², more preferably 10 to 100 mJ/cm². On use of EB, a pattern may be written directly or through a mask having a desired pattern, preferably in a dose of about 0.1 to 100 μC/cm², more preferably about 0.5 to 50 μC/cm². The molecular resist composition is suitable particularly in micropatterning using KrF excimer laser, ArF excimer laser, EB, EUV, X-ray, soft X-ray, γ-ray or synchrotron radiation, especially EB or EUV.

Since the molecular resist composition is adapted to form a pattern or image via a structural change of the sulfonium salt during exposure, the post-exposure bake (PEB) as is necessary for chemically amplified resist compositions is not always necessary. If PEB is involved, the resist film after exposure is baked on a hotplate or in an oven preferably at 30 to 120° C. for 10 seconds to 30 minutes, more preferably at 60 to 100° C. for 30 seconds to 20 minutes.

After the exposure or PEB, the resist film is developed with a developer in the form of an aqueous base solution for 3 seconds to 3 minutes, preferably 5 seconds to 2 minutes by conventional techniques such as dip, puddle and spray techniques. A typical developer is a 0.1 to 10 wt %, preferably 2 to 5 wt % aqueous solution of tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH), or tetrabutylammonium hydroxide (TBAH). In this way, a desired resist pattern is formed on the substrate. Since the molecular resist composition is of negative tone, the exposed region of the resist film is insolubilized and the unexposed region is dissolved away.

After the development in alkaline developer, the resist film is rinsed with pure water and dried by spin drying. Use of a rinse fluid containing a surfactant or supercritical rinsing with carbon dioxide is effective for reducing the stress applied onto the pattern during drying for thereby preventing pattern collapse.

The molecular resist composition may be subjected to negative tone development via organic solvent development to form a negative tone pattern. The organic solvent used as the developer is preferably selected from 2-octanone, 2-nonanone, 2-heptanone, 3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone, diisobutyl ketone, methylcyclohexanone, acetophenone, methylacetophenone, propyl acetate, butyl acetate, isobutyl acetate, pentyl acetate, isopentyl acetate, butenyl acetate, propyl formate, butyl formate, isobutyl formate, pentyl formate, isopentyl formate, methyl valerate, methyl pentenoate, methyl crotonate, ethyl crotonate, methyl propionate, ethyl propionate, ethyl 3-ethoxypropionate, methyl lactate, ethyl lactate, propyl lactate, butyl lactate, isobutyl lactate, pentyl lactate, isopentyl lactate, methyl 2-hydroxyisobutyrate, ethyl 2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, phenyl acetate, benzyl acetate, methyl phenylacetate, ethyl phenylacetate, benzyl formate, phenylethyl formate, methyl 3-phenylpropionate, benzyl propionate, and 2-phenylethyl acetate. These organic solvents may be used alone or in admixture of two or more.

At the end of development, the resist film is rinsed if necessary. As the rinsing liquid, a solvent which is miscible with the developer and does not dissolve the resist film is preferred. Suitable solvents include alcohols of 3 to 10 carbon atoms, ether compounds of 8 to 12 carbon atoms, alkanes, alkenes, and alkynes of 6 to 12 carbon atoms, and aromatic solvents. Specifically, suitable alcohols of 3 to 10 carbon atoms include n-propyl alcohol, isopropyl alcohol, 1-butyl alcohol, 2-butyl alcohol, isobutyl alcohol, tert-butyl alcohol, 1-pentanol, 2-pentanol, 3-pentanol, tert-pentyl alcohol, neopentyl alcohol, 2-methyl-1-butanol, 3-methyl-1-butanol, 3-methyl-3-pentanol, cyclopentanol, 1-hexanol, 2-hexanol, 3-hexanol, 2,3-dimethyl-2-butanol, 3,3-dimethyl-1-butanol, 3,3-dimethyl-2-butanol, 2-ethyl-1-butanol, 2-methyl-1-pentanol, 2-methyl-2-pentanol, 2-methyl-3-pentanol, 3-methyl-1-pentanol, 3-methyl-2-pentanol, 3-methyl-3-pentanol, 4-methyl-1-pentanol, 4-methyl-2-pentanol, 4-methyl-3-pentanol, cyclohexanol, and 1-octanol. Suitable ether compounds of 8 to 12 carbon atoms include di-n-butyl ether, diisobutyl ether, di-sec-butyl ether, di-n-pentyl ether, diisopentyl ether, di-sec-pentyl ether, di-tert-pentyl ether, and di-n-hexyl ether. Suitable alkanes of 6 to 12 carbon atoms include hexane, heptane, octane, nonane, decane, undecane, dodecane, methylcyclopentane, dimethylcyclopentane, cyclohexane, methylcyclohexane, dimethylcyclohexane, cycloheptane, cyclooctane, and cyclononane. Suitable alkenes of 6 to 12 carbon atoms include hexene, heptene, octene, cyclohexene, methylcyclohexene, dimethylcyclohexene, cycloheptene, and cyclooctene. Suitable alkynes of 6 to 12 carbon atoms include hexyne, heptyne, and octyne. Suitable aromatic solvents include toluene, xylene, ethylbenzene, isopropylbenzene, tert-butylbenzene and mesitylene.

Rinsing is effective for minimizing the risks of resist pattern collapse and defect formation. However, rinsing is not essential. If rinsing is omitted, the amount of solvent used may be reduced.

EXAMPLES

Examples of the invention are given below by way of illustration and not by way of limitation. The abbreviation “pbw” is parts by weight. Analysis is made by IR spectroscopy, NMR spectroscopy, and time-of-flight mass spectrometry (TOF-MS) using analytic instruments as shown below.

IR: NICOLET 6700 by Thermo Fisher Scientific Inc.

¹H-NMR: ECA-500 by JEOL Ltd.

MALDI TOF-MS: 53000 by JEOL Ltd.

[1] Synthesis of Sulfonium Salts Synthesis Example 1-1 Synthesis of PAG-1

In nitrogen atmosphere, 43.1 g of reactant M-1, 42.5 g of triethylamine, and 1.22 g of 4-dimethylaminopyridine were dissolved in 431 g of methylene chloride. The reaction system was cooled below 10° C. whereupon 55.8 g of methacrylic anhydride was added dropwise. At the end of addition, the reaction solution was aged at 20° C. for 12 hours. After aging, the reaction solution was cooled whereupon 200 g of saturated sodium bicarbonate water was added dropwise to quench the reaction. This was followed by conventional aqueous work-up, solvent distillation, and recrystallization from diisopropyl ether. PAG-1 was obtained as white crystals. Amount 58.1 g, yield 92%.

PAG-1 was analyzed by spectroscopy, with the data shown below. FIG. 1 is the ¹H-NMR/DMSO-d₆ spectrum of PAG-1.

IR (D-ATR): 3371, 2973, 2927, 1737, 1677, 1637, 1608, 1477, 1454, 1379, 1317, 1293, 1276, 1231, 1182, 1120, 1038, 1011, 947, 877, 807, 714, 650, 613, 583, 489 cm⁻¹

MALDI-TOFMS

-   -   Positive M⁺ 599 (corresponding to C₃₆H₃₉O₆S⁺)     -   Negative M⁻ 35 (corresponding to Cl⁻)

Synthesis Example 1-2 Synthesis of PAG-2

PAG-2 was synthesized by the same procedure as Synthesis Example 1-1 aside from changing methacrylic anhydride to acrylic chloride. Amount 14.4 g, yield 89%.

PAG-2 was analyzed by spectroscopy, with the data shown below. FIG. 2 is the ¹H-NMR/DMSO-d₆ spectrum of PAG-2.

IR (D-ATR): 3607, 3366, 2974, 2926, 1741, 1633, 1577, 1476, 1403, 1293, 1277, 1243, 1180, 1139, 1102, 1016, 982, 901, 802, 718, 667, 611, 588 cm⁻¹

MALDI-TOFMS

-   -   Positive M⁺ 557 (corresponding to C₃₃H₃₃O₆S⁺)     -   Negative M⁻ 35 (corresponding to Cl⁻)

Synthesis Example 1-3 Synthesis of PAG-3

A reactor in nitrogen atmosphere was charged with 14.6 g of PAG-1, 10.3 g of reactant M-2, 40 g of methylene chloride, and 40 g of water, which were stirred for 30 minutes. An organic layer was taken out by separatory operation and washed 5 times with 40 g of water. By distilling off the solvent from the organic layer and recrystallizing from diisopropyl ether, PAG-3 was obtained as white crystals. Amount 9.3 g, yield 75%.

PAG-3 was analyzed by spectroscopy, with the data shown below. FIG. 3 is the ¹H-NMR/DMSO-d₆ spectrum of PAG-3.

IR (D-ATR): 3506, 2929, 2855, 1737, 1637, 1476, 1452, 1379, 1290, 1182, 1110, 1036, 1011, 990, 947, 877, 807, 755, 657, 609, 582, 544, 528, 457 cm⁻¹

MALDI-TOFMS

-   -   Positive M⁺599 (corresponding to C₃₆H₃₉O₆S⁺)     -   Negative M⁻ 385 (corresponding to C₁₄H₂₅O₆S₃ ⁻)

Synthesis Example 1-4 Synthesis of PAG-4

PAG-4 was synthesized by the same procedure as Synthesis Example 1-3 aside from changing PAG-1 to PAG-2. Amount 10.3 g, yield 84%.

PAG-4 was analyzed by spectroscopy, with the data shown below. FIG. 4 is the ¹H-NMR/DMSO-d₆ spectrum of PAG-4.

IR (D-ATR): 3489, 3046, 2933, 2856, 1743, 1633, 1579, 1477, 1451, 1401, 1379, 1293, 1242, 1180, 1136, 1107, 1015, 983, 959, 901, 802, 764, 718, 656, 628, 610, 586, 544, 528, 458, 403 cm⁻¹

MALDI-TOFMS

-   -   Positive M⁺557 (corresponding to C₃₃H₃₃O₆S⁺)     -   Negative M⁻ 385 (corresponding to C₁₄H₂₅O₆S₃ ⁻)

Synthesis Example 1-5 Synthesis of PAG-5

PAG-5 was synthesized by the same procedure as Synthesis Example 1-4 aside from changing reactant M-2 to sodium styrenesulfonate. Amount 13.7 g, yield 90%.

PAG-5 was analyzed by spectroscopy, with the data shown below. FIG. 5 is the ¹H-NMR/DMSO-d₆ spectrum of PAG-5.

IR (D-ATR): 3048, 2972, 2926, 1743, 1633, 1477, 1399, 1294, 1277, 1240, 1206, 1180, 1137, 1104, 1060, 1033, 1010, 987, 975, 902, 841, 804, 714, 674, 611, 584, 553 cm⁻¹

MALDI-TOFMS

-   -   Positive M⁺557 (corresponding to C₃₃H₃₃O₆S⁺)     -   Negative M⁻ 183 (corresponding to C₈H₇O₃S⁻)

Synthesis Example 1-6 Synthesis of PAG-6

PAG-6 was synthesized by the same procedure as Synthesis Example 1-3 aside from changing reactant M-2 to sodium styrenesulfonate. Amount 16.1 g, yield 96%.

PAG-6 was analyzed by spectroscopy, with the data shown below. FIG. 6 is the ¹H-NMR/DMSO-d₆ spectrum of PAG-6.

IR (D-ATR): 3453, 2972, 2927, 1729, 1637, 1475, 1452, 1379, 1317, 1294, 1276, 1216, 1202, 1180, 1119, 1034, 1010, 943, 891, 843, 810, 714, 675, 612, 582, 557, 505, 487 cm⁻¹

MALDI-TOFMS

-   -   Positive M⁺599 (corresponding to C₃₆H₃₉O₆S⁺)     -   Negative M⁻ 183 (corresponding to C₈H₇O₃S⁻)

Synthesis Example 1-7 Synthesis of PAG-7

PAG-7 was synthesized by the same procedure as Synthesis Example 1-4 aside from changing reactant M-2 to reactant M-3. Amount 15.9 g, yield 86%.

PAG-7 was analyzed by spectroscopy, with the data shown below. FIG. 7 is the ¹H-NMR/DMSO-d₆ spectrum of PAG-7.

IR (D-ATR): 3480, 3050, 2971, 2928, 1743, 1633, 1608, 1476, 1404, 1377, 1328, 1245, 1181, 1139, 1101, 1071, 1015, 991, 902, 860, 840, 802, 777, 712, 667, 642, 610, 574, 552, 521 cm⁻¹

MALDI-TOFMS

-   -   Positive M⁺557 (corresponding to C₃₃H₃₃O₆S⁺)     -   Negative M⁻ 359 (corresponding to C₁₂H₈F₅O₅S⁻)

Synthesis Example 1-8 Synthesis of PAG-8

PAG-8 was synthesized by the same procedure as Synthesis Example 1-1 aside from changing reactant M-1 to reactant M-4. Amount 32.1 g, yield 92%.

PAG-8 was analyzed by spectroscopy, with the data shown below. FIG. 8 is the ¹H-NMR/DMSO-d₆ spectrum of PAG-8.

IR (D-ATR): 3379, 2974, 2926, 1735, 1636, 1477, 1446, 1379, 1317, 1293, 1277, 1182, 1120, 1040, 1011, 947, 878, 808, 751, 714, 685, 649, 613, 568, 522, 492 cm⁻¹

MALDI-TOFMS

-   -   Positive M⁺487 (corresponding to C₃₀H₃₁O₄S⁺)     -   Negative M⁻ 35 (corresponding to Cl⁻)

Synthesis Example 1-9 Synthesis of PAG-9

PAG-9 was synthesized by the same procedure as Synthesis Example 1-3 aside from changing PAG-1 to PAG-8. Amount 16.5 g, yield 89%.

PAG-9 was analyzed by spectroscopy, with the data shown below. FIG. 9 is the ¹H-NMR/DMSO-d₆ spectrum of PAG-9.

IR (D-ATR): 3506, 3056, 2929, 2854, 1738, 1637, 1476, 1448, 1379, 1289, 1258, 1183, 1110, 1012, 990, 948, 892, 852, 807, 754, 713, 686, 657, 609, 582, 544, 528, 456 cm⁻¹

MALDI-TOFMS

-   -   Positive M⁺ 487 (corresponding to C₃₀H₃₁O₄S⁺)     -   Negative M⁻ 385 (corresponding to C₁₄H₂₅O₆S₃ ⁻)

Synthesis Examples 1-10 to 1-20 Synthesis of PAG-10 to PAG-20

PAG-10 to PAG-20 were synthesized by various organic synthesis reactions.

[2] Synthesis of Base Polymer for Comparative Resist Composition Comparative Synthesis Example 1 Synthesis of Polymer P-1

In nitrogen atmosphere, 27.8 g of p-hydroxystyrene, 72.2 g of 1-methylcyclopentyl methacrylate, and 6.08 g of dimethyl 2,2′-azobisisobutyrate were dissolved in 155 g of PGMEA. In nitrogen atmosphere and with stirring, the solution was added dropwise to 78 g of PGMEA at 80° C. over 6 hours. At the end of addition, the solution was stirred for 2 hours while maintaining the temperature of 80° C. The solution was cooled to room temperature, after which it was added dropwise to 3,000 g of n-hexane for precipitation. The solid precipitate was filtered and dried in vacuum at 50° C. for 20 hours, obtaining Polymer P-1 in white powder form. Amount 85 g and yield 85%.

Comparative Synthesis Example 2 Synthesis of Polymer P-2

Polymer P-2 was synthesized by the same procedure as in Comparative Synthesis Example 1 aside from changing the type and mixing ratio of monomers.

[3] Preparation of Resist Composition Examples 1-1 to 1-20 and Comparative Examples 1-1 to 1-5

Molecular resist compositions (R-1 to R-20) were prepared by dissolving a sulfonium salt (PAG-1 to PAG-20) in a solvent in accordance with the formulation shown in Table 1, and filtering through a Teflon® filter with a pore size of 0.2 μm. Comparative resist compositions (CR-1 to CR-5) were prepared by mixing a polymer, photoacid generator, sensitivity adjusting agent, surfactant and solvent in accordance with the formulation shown in Table 1, and filtering through a Teflon® filter with a pore size of 0.2 μm.

TABLE 1 Base Photoacid Nonionic Sensitivity Resist polymer generator monomer adjusting Surfactant Solvent 1 Solvent 2 composition (pbw) (pbw) (pbw) agent (pbw) (pbw) (pbw) (pbw) Example 1-1 R-1 — PAG-1 — — — PGMEA DAA (100) (1,500) (600) 1-2 R-2 — PAG-2 — — — PGMEA DAA (100) (1,500) (600) 1-3 R-3 — PAG-3 — — — PGMEA DAA (100) (1,500) (600) 1-4 R-4 — PAG-4 — — — PGMEA DAA (100) (1,500) (600) 1-5 R-5 — PAG-5 — — — PGMEA DAA (100) (1,500) (600) 1-6 R-6 — PAG-6 — — — PGMEA DAA (100) (1,500) (600) 1-7 R-7 — PAG-7 — — — PGMEA DAA (100) (1,500) (600) 1-8 R-8 — PAG-8 — — — PGMEA DAA (100) (1,500) (600) 1-9 R-9 — PAG-9 — — — PGMEA DAA (100) (1,500) (600) 1-10 R-10 — PAG-10 — — — PGMEA DAA (100) (1,500) (600) 1-11 R-11 — PAG-11 — — — PGMEA DAA (100) (1,500) (600) 1-12 R-12 — PAG-12 — — — PGMEA DAA (100) (1,500) (600) 1-13 R-13 — PAG-13 — — — PGMEA DAA (100) (1,500) (600) 1-14 R-14 — PAG-14 — — — PGMEA DAA (100) (1,500) (600) 1-15 R-15 — PAG-15 — — — PGMEA DAA (100) (1,500) (600) 1-16 R-16 — PAG-16 — — — PGMEA DAA (100) (1,500) (600) 1-17 R-17 — PAG-17 — — — PGMEA DAA (100) (1,500) (600) 1-18 R-18 — PAG-18 — — — PGMEA DAA (100) (1,500) (600) 1-19 R-19 — PAG-19 — — — PGMEA DAA (100) (1,500) (600) 1-20 R-20 — PAG-20 — — — PGMEA DAA (100) (1,500) (600) Comparative 1-1 CR-1 — PAG-A — — — PGMEA DAA Example (108) (1,500) (600) 1-2 CR-2 — PAG-B — — — PGMEA DAA (100) (1,500) (600) 1-3 CR-3 P-1 PAG-C — Q-A SF-1 PGMEA DAA (65) (19) (1.3) (0.01) (1,500) (600) 1-4 CR-4 P-2 — — Q-B SF-1 PGMEA DAA (65) (4.0) (0.01) (1,500) (600) 1-5 CR-5 — PAG-C Y-1 Q-A SF-1 PGMEA DAA (10) (100) (1.3) (0.01) (1,500) (600)

The photoacid generators (PAG-A to PAG-C), sensitivity adjusting agent (Q-A, Q-B), nonionic monomer (Y-1), surfactant (SF-1), and solvents in Table 1 are identified below.

SF-1: PF636 (Omnova Solutions Inc.) Solvent:

-   -   PGMEA (propylene glycol monomethyl ether acetate)     -   DAA (diacetone alcohol)

[4] EB Lithography Test Examples 2-1 to 2-20 and Comparative Examples 2-1 to 2-5

An antireflective coating of 60 nm thick (DUV-42 by Nissan Chemical Industries, Ltd.) was formed on a silicon substrate. Each of the resist compositions (R-1 to R-20, CR-1 to CR-5) was spin coated on the ARC, and baked on a hotplate at 100° C. for 60 seconds to form a resist film of 50 nm thick. The resist film was exposed to EB on an EB lithography system (ELS-F125, Elionix Co., Ltd., accelerating voltage 125 kV), baked (PEB) on a hotplate at the temperature shown in Table 2 for 60 seconds, and developed in a 2.38 wt % TMAH aqueous solution for 30 seconds to form a pattern. Examples 2-1 to 2-20, Comparative Examples 2-2 and 2-5 showed negative tone performance in that the resist film in the exposed region was left. Comparative Examples 2-3 and 2-4 showed positive tone performance in that the resist film in the unexposed region was left. As a result, line-and-space (LS) patterns of negative or positive tone having a space width of 40 nm and a pitch of 80 nm were obtained. In Comparative Example 2-1, no pattern formation was confirmed.

The LS pattern was observed under an electron microscope CD-SEM (CG-5000 by Hitachi High-Technologies Corp.). The LS pattern was evaluated for sensitivity, LWR, and maximum resolution by the following methods. The results are shown in Table 2.

Evaluation of Sensitivity

The optimum dose Eop (μC/cm²) which provided a LS pattern with a space width of 40 nm and a pitch of 80 nm was determined and reported as sensitivity.

Evaluation of LWR

For the LS pattern formed by exposure in the optimum dose Eop, the space width was measured at longitudinally spaced apart 10 points, from which a 3-fold value (3σ) of standard deviation (σ) was determined and reported as LWR. A smaller value of 3σ indicates a pattern having a lower roughness and more uniform space width.

Evaluation of Maximum Resolution

The minimum line width (nm) of the LS pattern which remains separate at the optimum dose Eop is reported as maximum resolution.

TABLE 2 PEB Maximum Resist temp. Eop LWR resolution composition (° C.) (μC/cm²) (nm) (nm) Example 2-1 R-1 60 200 4.5 28 2-2 R-2 60 210 4.5 28 2-3 R-3 60 180 4.4 26 2-4 R-4 60 130 4.4 26 2-5 R-5 60 60 4.1 26 2-6 R-6 60 90 4.0 24 2-7 R-7 60 70 4.2 24 2-8 R-8 60 250 4.5 28 2-9 R-9 60 200 4.4 26 2-10 R-10 60 180 4.4 26 2-11 R-11 60 140 4.4 26 2-12 R-12 60 130 4.3 26 2-13 R-13 60 130 4.3 26 2-14 R-14 60 180 4.4 26 2-15 R-15 60 70 4.2 26 2-16 R-16 60 120 4.1 24 2-17 R-17 60 120 4.4 26 2-18 R-18 60 100 4.2 24 2-19 R-19 60 170 4.4 26 2-20 R-20 60 110 4.3 24 Comparative 2-1 CR-1 60 — — — Example 2-2 CR-2 60 280 4.8 30 2-3 CR-3 80 560 6.2 40 2-4 CR-4 80 500 5.4 40 2-5 CR-5 85 400 6.6 45

It is evident from Table 2 that the molecular resist compositions within the scope of the invention are improved in sensitivity, LWR and maximum resolution over polymeric positive resist compositions and positive resist compositions using a nonionic monomer when negative patterns are formed by EB lithography and alkaline solvent development.

[5] EUV Lithography Test Examples 3-1 to 3-20 and Comparative Examples 3-1 to 3-5

Each of the resist compositions (R-1 to R-20, CR-1 to CR-5) was spin coated on a silicon substrate having a 20-nm coating of silicon-containing spin-on hard mask SHB-A940 (Shin-Etsu Chemical Co., Ltd., silicon content 43 wt %) and prebaked on a hotplate at 100° C. for 60 seconds to form a resist film of 40 nm thick. Using an EUV scanner NXE3300 (ASML, NA 0.33, σ 0.9, 90° dipole illumination), the resist film was exposed to EUV through a mask bearing a 1:1 LS pattern with a line width of 22 nm. The resist film was baked (PEB) on a hotplate at the temperature shown in Table 3 for 60 seconds and developed in a 2.38 wt % TMAH aqueous solution for 30 seconds to form a pattern. In Examples 3-1 to 3-20 and Comparative Examples 3-2 and 3-5, the resist film in the exposed region was left. In Comparative Examples 3-3 and 3-4, the resist film in the unexposed region was left. As a result, LS patterns of negative or positive tone having a space width of 22 nm and a pitch of 44 nm were obtained. In Comparative Example 3-1, no pattern formation was confirmed.

The LS pattern was observed under an electron microscope CD-SEM (CG-5000 by Hitachi High-Technologies Corp.). The LS pattern was evaluated for sensitivity, LWR, and maximum resolution by the following methods. The results are shown in Table 3.

Evaluation of Sensitivity

The optimum dose Eop (mJ/cm²) which provided a LS pattern with a space width of 22 nm and a pitch of 44 nm was determined and reported as sensitivity.

Evaluation of LWR

For the LS pattern formed by exposure in the optimum dose Eop, the space width was measured at longitudinally spaced apart 10 points, from which a 3-fold value (3σ) of standard deviation (σ) was determined and reported as LWR. A smaller value of 3σ indicates a pattern having a lower roughness and more uniform space width.

Evaluation of Maximum Resolution

The minimum line width (nm) of the LS pattern which remains separate at the optimum dose Eop is reported as maximum resolution.

TABLE 3 PEB Maximum Resist temp. Eop LWR resolution composition (° C.) (mJ/cm²) (nm) (nm) Example 3-1 R-1 60 200 2.8 14 3-2 R-2 60 210 2.8 14 3-3 R-3 60 180 2.7 14 3-4 R-4 60 130 2.7 14 3-5 R-5 60 60 2.6 14 3-6 R-6 60 90 2.5 12 3-7 R-7 60 70 2.6 14 3-8 R-8 60 250 2.8 14 3-9 R-9 60 200 2.7 14 3-10 R-10 60 180 2.7 14 3-11 R-11 60 140 2.7 14 3-12 R-12 60 130 2.7 14 3-13 R-13 60 130 2.7 14 3-14 R-14 60 180 2.8 14 3-15 R-15 60 70 2.5 12 3-16 R-16 60 130 2.4 14 3-17 R-17 60 120 2.7 14 3-18 R-18 60 100 2.4 12 3-19 R-19 60 170 2.7 14 3-20 R-20 60 120 2.6 14 Comparative 3-1 CR-1 60 — — — Example 3-2 CR-2 60 30 3 16 3-3 CR-3 80 52 4.3 22 3-4 CR-4 80 40 3.8 20 3-5 CR-5 85 36 4.1 24

It is evident from Table 3 that as in the case of EB lithography, the molecular resist compositions within the scope of the invention are improved in sensitivity, LWR and maximum resolution over polymeric positive resist compositions and positive resist compositions using a nonionic monomer when negative patterns are formed by EUV lithography and alkaline solvent development.

Japanese Patent Application No. 2021-179526 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A molecular resist composition comprising a sulfonium salt having the formula (1) or (2) and an organic solvent, the composition being free of a base polymer,

wherein n is an integer of 1 to 3, A¹ is a polymerizable functionality-bearing C₂-C₂₀ hydrocarbyl group which may contain a heteroatom, A² is a polymerizable functionality-bearing group which forms a C₄-C₂₀ aliphatic ring with two carbon atoms in Ar^(1B), and the aliphatic ring may contain a heteroatom, Ar^(1A) is a C₆-C₂₀ arylene group in which some or all of the hydrogen atoms on the aromatic ring may be substituted by halogen or a C₁-C₂₀ hydrocarbyl moiety which may contain a heteroatom, Ar^(1B) is a C₆-C₂₀ trivalent aromatic hydrocarbon group in which some or all of the hydrogen atoms on the aromatic ring may be substituted by halogen or a C₁-C₂₀ hydrocarbyl moiety which may contain a heteroatom, Ar² is a C₆-C₂₀ aryl group in which some or all of the hydrogen atoms on the aromatic ring may be substituted by halogen or a C₁-C₂₀ hydrocarbyl moiety which may contain a heteroatom, two Ar^(1A), two Ar^(1B), two Ar², Ar^(1A) and Ar², or Ar^(1B) and Ar² may bond together to form a ring with the sulfur atom to which they are attached, and X⁻ is a counter anion.
 2. The molecular resist composition of claim 1 wherein A¹ is acryloyloxy, methacryloyloxy, a C₃-C₂₀ cycloalkenyl group, C₃-C₂₀ cycloalkenyloxy group, C₃-C₂₀ cycloalkenylcarbonyloxy group, C₂-C₂₀ alkenyl group, or C₂-C₂₀ alkenyloxy group, the cycloalkenyl, cycloalkenyloxy, cycloalkenylcarbonyloxy, alkenyl and alkenyloxy groups may contain a heteroatom, A² is a group which forms a C₄-C₂₀ cycloalkene ring or C₄-C₂₀ multiple ring having one double bond, with two carbon atoms in Ar^(1B), the cycloalkene ring and multiple ring may contain a heteroatom.
 3. The molecular resist composition of claim 1 wherein X⁻is a halide ion, nitrate ion, hydrogensulfate ion, hydrogencarbonate ion, tetraphenylborate ion, or an anion having any one of the formulae (X-1) to (X-7):

wherein k is an integer of 1 to 4, Rf¹ and Rf² are each independently hydrogen, fluorine or a C₁-C₆ fluorinated alkyl group, excluding that all Rf¹ and Rf² are hydrogen at the same time, R¹¹ is hydrogen, halogen, hydroxy, or a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom, R¹² is hydrogen, halogen, hydroxy, or a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom, exclusive of a hydrocarbyl group in which hydrogen on α- or β-carbon relative to the sulfo group is substituted by fluorine or fluoroalkyl, R²¹ is hydrogen, halogen, hydroxy, or a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom, R²² is hydrogen, halogen, hydroxy, or a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom, exclusive of a hydrocarbyl group in which hydrogen on α- or β-carbon relative to the carboxy group is substituted by fluorine or fluoroalkyl, R³¹ and R³² are each independently a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom, R⁴¹ to R⁴³ are each independently a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom, R⁵¹ is fluorine or a C₁-C₁₀ fluorinated hydrocarbyl group which may contain hydroxy, ether bond or ester bond, R⁵² is hydrogen or a C₁-C₂₀ hydrocarbyl group which may contain hydroxy, ether bond or ester bond, R⁵¹ and R⁵² may bond together to form a ring with the atoms to which they are attached.
 4. The molecular resist composition of claim 1 wherein the anion having any one of formulae (X-1) to (X-7) contains a polymerizable functional group.
 5. The molecular resist composition of claim 1, further comprising a radical trapping agent.
 6. The molecular resist composition of claim 1, further comprising a surfactant.
 7. A pattern forming process comprising the steps of applying the molecular resist composition of claim 1 onto a substrate to form a resist film thereon, exposing the resist film to high-energy radiation, and developing the exposed resist film in a developer.
 8. The process of claim 7 wherein the developing step uses an alkaline aqueous solution as the developer to form a positive tone pattern wherein the resist film in the exposed region is dissolved away and the resist film in the unexposed region is not dissolved.
 9. The process of claim 7 wherein the developing step uses an organic solvent as the developer to form a negative tone pattern wherein the resist film in the unexposed region is dissolved away and the resist film in the exposed region is not dissolved.
 10. The process of claim 9 wherein the organic solvent is at least one solvent selected from among 2-octanone, 2-nonanone, 2-heptanone, 3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone, diisobutyl ketone, methylcyclohexanone, acetophenone, methylacetophenone, propyl acetate, butyl acetate, isobutyl acetate, pentyl acetate, butenyl acetate, isopentyl acetate, propyl formate, butyl formate, isobutyl formate, pentyl formate, isopentyl formate, methyl valerate, methyl pentenoate, methyl crotonate, ethyl crotonate, methyl propionate, ethyl propionate, ethyl 3-ethoxypropionate, methyl lactate, ethyl lactate, propyl lactate, butyl lactate, isobutyl lactate, pentyl lactate, isopentyl lactate, methyl 2-hydroxyisobutyrate, ethyl 2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, phenyl acetate, benzyl acetate, methyl phenylacetate, benzyl formate, phenylethyl formate, methyl 3-phenylpropionate, benzyl propionate, and 2-phenylethyl acetate.
 11. The process of claim 7 wherein the high-energy radiation is EB or EUV. 